1. Field of the Invention
This invention relates to enhanced oil recovery processes wherein one or more fluids are injected into a subterranean petroleum reservoir for the purpose of mobilizing and driving the petroleum liquids contained therein towards production wells. More particularly, this invention relates to an improved well pattern for the location of these injection and production wells.
2. Description of the Prior Art
The development of different well patterns for petroleum recovery operations has reached a relatively sophisticated level. Perhaps the simplest is the square grid pattern which is initially employed during the primary recovery operation stage in the development of the oil field wherein all of the wells are utilized as producing wells. Then, during secondary recovery operations, some of the producing wells are converted into water injection wells for the purpose of recovering more petroleum from the reservoir. However, well pattern development has not stopped with the square grid pattern. Other common well patterns include the five spot pattern in which a central producing well is located within each individual grid of the square grid array with injection wells on the corners of the square. Other patterns include inverted five spot, the nine spot in which the central well is an injection well with the producing wells located on the corners, offset lines, and a host of others. Each of these individual systems can be further developed by switching wells within the pattern from in injection mode to a production mode and vice versa. Furthermore, individual wells within each pattern can be shut in. Each variation is employed for the purpose of increasing the areal sweep efficiency of the pattern. This areal sweep efficiency is a measure of the area actually swept by fluids injected through the injections wells which then flow through the formation and are produced from the production wells. In relation to the total area bounded by the well pattern, this areal sweep efficiency is measured in the horizontal plane and is usually expressed as a percentage. U.S. Pat. Nos. 3,845,817, 3,882,922, 3,874,449 and 3,877,521 to Altamira and Hoyt are representative references.
However, most if not all of the developments in this area have been directed towards improving only the areal sweep efficiency. To do so is to neglect other important factors such as vertical sweep efficiency, which can play a very important role in the overall effectiveness of a given enhanced oil recovery operation. Vertical sweep efficiency measures the areas swept within a formation as a function of the total vertical cross section of the producing interval, and is measured in a vertical rather than a horizontal plane. Such a measurement becomes quite critical in situations where the petroleum sought to be recovered, such as heavy oils and the bitumen contained in tar sands, is swept by fluids of a much lower specific gravity such as water, steam or hydrocarbon gases, carbon dioxide or other such fluids. Steam injection programs are particularly troublesome in this respect because the steam tends to rise immediately to the top of the producing interval, producing a condition known as override. The steam will commonly preferentially flow through this override channel to the production well, leaving the majority of the producing interval unswept. This steam override effect produces quite low vertical sweep efficiencies and therefore low efficiencies for the enhanced oil recovery system as a whole. Typical references addressed to this problem are U.S. Pat. Nos. 4,166,501; 4,166,502; 4,166,503; 4,166,504; 4,177,752 and pending application Ser. No. 141,243 filed Apr. 17, 1980. Nevertheless, there remains a long felt need in the art for an improved well pattern, which can produce both a high areal sweep efficiency and a high vertical sweep efficiency.